Nickel-iron battery with a chemically pre-formed (CPF) iron negative electrode

ABSTRACT

Provided is a Ni—Fe battery comprising an iron electrode which is preconditioned prior to any charge-discharge cycle. The preconditioned iron electrode used in the Ni—Fe battery is prepared by first fabricating an electrode comprising an iron active material, and then treating the surface of the electrode with an oxidant to thereby create an oxidized surface.

RELATED APPLICATIONS

The present application claims priority to provisional applications U.S.61/874,177 filed on Sep. 5, 2013 and U.S. 61/901,199 filed on Nov. 7,2013, with both applications herein incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

Field of Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of rechargeable batteries using iron electrodes.

Related Art

The nickel iron (Ni—Fe) battery was independently developed by Edison inthe United States and by Junger in Sweden in 1901. It was industriallyimportant from its introduction until the 1970's when batteries withsuperior specific energy and energy density replaced Ni—Fe batteries inmany applications.

However, Ni—Fe batteries have many advantages over other batterychemistries. The Ni—Fe battery is a very robust battery which is verytolerant of abuse such as overcharge and overdischarge and can have avery long life. It is often used in backup situations where it can becontinuously trickle-charged and last more than 20 years. Additionally,the iron active material is much less expensive than active materialsused in other alkaline battery systems such as NiMH or in non-aqueousbatteries such as Li Ion. However, the low specific energy, low energydensity, and poor power have limited the applications of this batterysystem.

The Ni—Fe battery is a rechargeable battery having a nickel(III)oxy-hydroxide positive electrode and an iron negative electrode, with analkaline electrolyte such as potassium hydroxide. The overall cellreaction can be written as:2NiOOH+Fe+2H₂O

2Ni(OH)₂+Fe(OH)₂  (1)

Rechargeable batteries often require several charge-discharge cyclesprior to achieving optimum performance. During these early cycles,critical surface films are formed on the electrode surfaces that affectthe performance of the cell during later cycling. These early cycles arecommonly termed formation cycles in the battery industry. In the case ofnickel-iron batteries (Ni—Fe), 30 to 60 formation cycles are typicallyneeded to achieve the full capacity of the cell. Formation cyclingsometimes requires cycling at varied temperature regimes whichcomplicates the process. This formation process is expensive, timeconsuming, consumes electrolyte which needs replacing, and generates asignificant amount of gas. Therefore, reducing the number of formationcycles and simplifying the formation process is a worthy goal.

Manohar et. al. in “Understanding the factors affecting the formation ofCarbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries”, J.Electrochem. Soc., 159, 12, (2012) A 2148-2155, reported that one reasonfor the long formation time could also be the poor wettability of theiron electrode and the inaccessibility of the pores of the iron by theelectrolyte. As the pores became more accessible the charge anddischarge process produced a progressively rougher surface resulting inan increase in electrochemically active surface area and dischargecapacity. Triton X-100, a surfactant, reduced the number of cyclesrequired to achieve higher capacity presumably because it improvedaccess of the electrolyte to the pores.

U.S. Pat. No. 3,507,696 teaches that a mixture of FeO and Fe₂O₃ powdersfused with sulfur at 120° C. yields an active material that may be usedin an aqueous slurry to impregnate sintered nickel fiber plaques thatcan used as a negative electrode in a Ni—Fe battery. Several formationcycles are needed to achieve high capacity.

It would be of benefit to the industry to have a battery comprising aniron electrode which is conditioned prior to any charge-discharge cycleso as to minimize the need for formation cycles.

SUMMARY OF THE INVENTION

Provided is a nickel-iron (Ni—Fe) battery which comprises a chemicallypre-formed (CPF) iron negative electrode. The CPF iron electrode isprepared by:

i) fabricating an electrode comprising an iron active material, and

ii) treating the surface of the electrode with an oxidant to therebycreate an oxidized surface. In one embodiment, the oxidant compriseswater, hydrogen peroxide, ozone, chlorine, nitric acid, hypochlorite,nitrous oxide, bromine, iodine, permanganate compounds, or sodiumperborate.

In another embodiment, provided is a nickel-iron battery comprised of anelectrode which comprises an iron active material, and which electrodehas been preconditioned prior to any charge-discharge cycle to have theaccessible surface of the iron material in the same oxidation state asdischarged iron negative electrode active material. In one embodiment,the oxidation state of the conditioned iron active material is +2,+2/+3, +3 or +4.

Among other factors, the present invention provides a nickel-ironbattery which contains a CPF iron anode, i.e., an iron anode conditionedprior to any charge-discharge cycle. The employment of this iron anodein the Ni—Fe battery addresses the mismatch in the state-of-charge (SOC)of the anode and cathode that is present during Ni—Fe cell assembly. Useof the present iron electrode in the Ni—Fe battery decreases the numberof cycles, and time to achieve cell formation, electrolyte consumption,hydrogen gas generated, and the amount of water needed to refill thecell. In general, the use leads to improved iron utilization in thebattery.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is an illustration of the interparticle contact between activematerial particles and the space between particles or pores that can befilled with an oxident to precondition the surface of the electrodeparticles.

FIG. 2 shows the capacity of Ni—Fe cells with and without preconditionediron electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises an improved Ni—Fe battery employing achemically preconditioned iron electrode. In one embodiment, the ironelectrode is comprised of a single conductive substrate coated with ironactive material on one or both sides. The battery may be prepared byconventional processing and construction employing a nickel oxyhydroxidepositive electrode, an alkaline electrolyte, and separator. The nickelelectrode may be of a sintered type well known in the art or may be of apasted type employing a foam or felt matrix. In one embodiment, theseparator is a polyolefin material. The battery electrolyte may compriseof a KOH solution or alternatively be a NaOH based electrolyte.

The chemically preconditioned iron electrode used in the NiFe battery ofthe present invention is prepared by chemically treating an iron metalelectrode after the electrode is assembled to provide a preconditionediron electrode. The preconditioned electrode may be prepared from astandard iron electrode used in Ni—Fe cells. These iron electrodes canbe comprised of iron particles or mixtures thereof with sulfur, nickel,or other metal powders, bonded to a substrate. In one embodiment, aconductive additive for the iron electrode comprises nickel, carbonblack or copper. In one embodiment, an additive of the iron electrodecomprises sulfur. In another embodiment, the coating of active materialof the iron electrode comprises a binder for the iron or iron activematerial, and additives. The binder is generally a polymer such as PVA,or a rubber. The use of a PVA binder has been found to be quitebeneficial and advantageous.

In one embodiment, the iron electrode comprises about 50-90 wt % ironpowder, and in another embodiment from about 75-85 wt % iron powder;from about 5-30 wt % nickel powder, and in another embodiment from about12-20 wt % nickel powder; from 0.5-5.0 wt % binder, and in anotherembodiment from about 2.0-5.0 wt % binder; and, from 0.25-2.0 wt %sulfur, and in another embodiment, from about 0.25-1.0 wt % sulfur. Inone embodiment, the iron electrode comprises about 80 wt % iron powder,about 16 wt % nickel powder, about 3.5 wt % binder and about 0.5 wt %sulfur powder.

In one embodiment, the iron electrode can comprise additionalconventional additives, such as pore formers. In general, the porosityof the iron electrode is in the range of from 15-50%, and in oneembodiment from 35-45%.

The substrate used in the electrode can be comprised of a conductivematerial such as carbon or metal. The substrate for the iron electrodeis generally a single layer of a conductive substrate coated on at leastone side with a coating comprising the iron active material. Both sidesof the substrate can be coated. In one embodiment, the coating on atleast one side comprises iron and additives comprised of sulfur,antimony, selenium, tellurium, nickel, bismuth, tin, or a mixturethereof. The substrate is generally a metal foil, metal sheet, metalfoam, metal mesh, woven metal or expanded metal. In one embodiment, thesubstrate for the iron electrode is comprised of a nickel plated steel.It is generally of porous construction such as that provided by a mesh,or grid of fibrous strands, or a perforated metal sheet. The ironelectrode can also be sintered.

The iron electrodes of the present invention are chemicallypreconditioned with oxidants that are able to oxidize the iron surface.These materials include but are not limited to: water, hydrogenperoxide, ozone, chlorine, nitric acid, hypochlorite, nitrous oxide,bromine, iodine, permanganate compounds, and sodium perborate. Oxidizingmaterials that are non-toxic and volatile and yield reduction or thermaldecomposition products that are also non-toxic and volatile arepreferred. Preferred oxidants are water and hydrogen peroxide. Water andsolutions used to pretreat the iron electrodes may or may not contain asurfactant. An example of a surfactant that may be used includes but isnot limited to Triton X-100. The treatment of the electrodes may beaccomplished by coating, dipping, spraying, or otherwise applyingoxidants or solutions containing the oxidant to the electrode. Theelectrode may also be preconditioned by exposing the electrode to anoxidizing gas. The electrode may be rinsed with water afterpreconditioning with the oxidant to remove the reduced form of theoxidant such as chloride. After rinsing, the electrode is then dried.

The length of time the electrodes are treated with the oxidizing gas canvary, but is generally until oxidation of the iron on the accessiblesurface of the electrode is observed. The temperature at which thetreatment is made is generally ambient, but it can be at highertemperatures. After the treatment, the electrode can be dried, ifneeded. It can be air dried or in an oven, for example. This is to makesure all of the oxidizing agent is removed.

In one embodiment, the treatment of the iron electrode is continueduntil the accessible surface of the iron material of the electrode is inthe same oxidation state as the electrode would in the discharged state.This is achieved by the oxidation treatment and can be determined usingconventional methods available.

While not wishing to be bound by theory, it is believed that nickel-ironbatteries may sometimes be assembled with the nickel cathode (positiveelectrode) in its discharged state and the iron anode (negativeelectrode) in its charged state. Thus, when the cell is assembled, thereis a mismatch between the state-of-charge (SOC) between the anode andcathode which is corrected during the formation process. During theformation process, it is believed that the low capacity of the earlycycles is due to the limited amount of discharge products (ie. Fe(OH)₂,Fe(OH)₃, and Fe₃O₄ depending on depth of discharge) that are formeduntil the proper conductivity, texture, and porosity of the ironelectrode is achieved. Consequently, the negative electrode is in ahigher SOC than the positive electrode for most of the formationprocess.

During the charge of a Ni—Fe cell there are typically two processes thatoccur at the anode surface, which are shown in Equations 1 and 2 below.Equation 1 is the desired conversion of discharge product, Fe(OH)₂, toiron metal. Equation 2 is the reduction of water to hydroxide andhydrogen gas. The two processes have very similar electrochemicalpotentials and both are usually active during the charge process.Fe(OH)₂+2e ⁻→Fe+2OH⁻ E°=−0.877 V  12H₂O+2e ⁻→H₂+2OH⁻ E°=−0.828 V  2

However, when the negative electrode is at high SOC as in formation, thereaction in Equation 2 is more dominant since there is too littleFe(OH)₂ or other iron compounds with iron in its +2 or +3 oxidationstate to accept current from the cathode. The reaction in Equation 2consumes the water in the electrolyte which needs to be replaced andgenerates significant amount of gas that can become trapped between theelectrodes, further hindering desired electrochemical reactions at theelectrode surfaces. Gas generation can cause loss of adhesion of theactive material to the electrode further damaging the electrode.

It is believed that chemically pretreating the electrode with oxidantsconverts areas of the electrode that are accessible by the alkalineelectrolyte, including pores, to iron compounds where iron is in its +2or +3 oxidation state that are capable of being reduced to iron metalwhen an electrochemical current is applied in a cell. The products ofthe pretreating of the iron electrode may be the same as the dischargeproducts on the iron electrode, or may be different. With some oxidants,rinsing may be necessary to remove the reduced form of the oxidant andconvert the iron salts to iron hydroxides and iron oxides. Followingthese treatments, the products may comprise independently or as amixture: Fe(OH)₂, Fe(OH)₃, Fe₃O₄, Fe₂O₃, FeO, and other iron oxides. Asa result, the mismatch in the SOC of the anode and cathode that ispresent during Ni—Fe cell assembly is minimized, if not avoided alltogether. Use of the present iron electrode thereby decreases the numberof cycles and time to achieve cell formation, electrolyte consumption,hydrogen gas generated, and the amount of water needed to refill thecell.

FIG. 1 shows a diagram of an electrode that has been preconditioned. Theiron particle active material, 1, retains interparticle contact, 2, andelectrical contact between the active materials and the substrate, 3, ismaintained. The surface of the electrode and the pores, 4, are able tobe contacted by the oxidant for preconditioning. Areas where there isinterparticle contact are not oxidized. Because the oxidation productsare electrically insulating, it is an advantage of this invention thatthe areas where there is interparticle contact are not oxidized,maintaining a conductive network between particles.

The present example is provided to further illustrate the presentinvention. It is not meant to be limiting.

Example

An aqueous slurry consisting of 80% iron, 16% nickel, and 0.5% sulfurpowders with 3.5% polyvinyl alcohol binder were pasted onto a perforatednickel sheet which was then dried. This sheet was then chemicallypreconditioned by brushing with deionized water and allowed to dry inair for 16 hours at room temperature followed by drying in an oven at190° C. for 15 minutes. A 16% weight gain was measured and a slightorange-brown color was observed on the surface of the electrode. Twosample electrodes were cut from this sheet and tabs were TIG welded tothe top uncoated area of the electrode. Two sample cells wereconstructed using these negative electrodes by placing the negativeelectrode between two commercial Histar sintered positive nickelhydroxide electrodes. Both the positive and negative electrodes werepocketed into a polypropylene separator. For comparison, two identicalcells were constructed from identical materials except that the negativeelectrodes were not chemically preconditioned. The test cells containingCPF iron negative electrodes and the control cells were subjected to anaccelerate life test at 55° C. with the following charge regime:

-   -   Cycle 1 (@ Room Temp): Charge: 1.0 A×1.5 hrs        -   Rest: 30 Min        -   Discharge: 0.1 A to 1.0 V        -   Rest: 30 Min    -   Cycle 2-100 (@ 55° C.): Charge: 1.0 A×1.5 hrs        -   Rest: 30 Min        -   Discharge: 0.1 A to 1.0 V        -   Rest: 30 Min

The cycling characteristics for cells prepared with pre-conditioned ironelectrodes is shown in FIG. 2. The cells with chemically pre-conditionednegative electrodes deliver a capacity of 140-160 mAh/g Fe after onlyfive cycles compared to a capacity of 120-135 mAh/g Fe after ten cyclesfor cells with negative electrodes that were not preconditioned.Furthermore, the overall capacity for cells with preconditionedelectrodes is between 17-19% higher for the life of the cell afterformation demonstrating a further advantage of this invention.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of variations, combination, and equivalents ofthe specific embodiment, method, and examples therein. The inventionshould therefore not be limited by the above described embodiment,method and examples, but by all embodiments and methods within the scopeand spirit of the inventions and the claims appended therein.

What is claimed is:
 1. A Ni—Fe battery comprising an electrode which hasnot been subject to a charge-discharge cycle, the electrode a singlelayer of conductive substrate coated on at least one side with a coatingcomprising an iron active material, a polyvinyl alcohol binder andsulfur, which electrode has an oxidized iron surface and areas ofinterparticle contact that are in an oxidized state different from theoxidized iron surface, and which electrode is prepared by a processcomprising: i) fabricating the electrode comprising an iron activematerial, and ii) treating the surface of the electrode with an oxidantto thereby create an oxidized surface prior to the battery beingsubjected to a charge-discharge cycle.
 2. The battery of claim 1,wherein the oxidation state of the oxidized iron surface is +2, +2/+3,+3 or +4.
 3. The battery of claim 1, wherein the electrode furthercomprises a conductive additive.
 4. The battery of claim 3, wherein theconductive additive comprises nickel, or copper or carbon black.
 5. Thebattery of claim 1, wherein the electrode comprises an iron activematerial comprising about: 50-90 wt % iron powder 5-30 wt % nickelpowder 0.5-5.0 wt % binder, and 0.25-20 wt % sulfur.
 6. The battery ofclaim 1, wherein the substrate of the electrode is comprised of nickelplated steel.
 7. The battery of claim 1, wherein the porosity of theelectrode is in the range of about 15-50%.
 8. The battery of claim 1,therein the oxidant comprises water, hydrogen peroxide, ozone, chlorine,nitric acid, hypochlorite, nitrous oxide, bromine, iodine, permanganatecompounds, or sodium perborate.
 9. The battery of claim 1, wherein theoxidant comprises water or hydrogen peroxide.
 10. The battery of claim1, wherein the oxidant comprises a gaseous oxidant.